Field-effect transistors (FETs) and other related insulated-gate electronic devices are mainstay components of metal oxide semiconductor (MOS) integrated circuits. A MOSFET generally consists of two closely spaced, doped regions in a substrate; namely, the source region and the drain region. The region between the two is the channel above which a thin insulation layer such as a gate oxide layer is formed. A gate conductor is formed from a gate material directly over the insulation layer directly above the channel and a voltage applied to the gate conductor affects the electronic properties of the channel region, whereby the MOSFET can control current flow between the source region and the drain region (e.g., is turned on and off).
PMOS devices are formed by implanting the substrate with a p-type dopant to form heavily doped p+ source and drain regions using a self-aligned process. Since the gate conductor is used in the self-aligned process, it is also implanted with a p-type dopant. Preferred examples of p-type dopant ions include B.sup.+ and BF.sub.2.sup.+. Among B.sup.+ and BF.sub.2.sup.+, BF.sub.2.sup.+ is preferred because of its larger atomic mass. The larger atomic mass results in a lesser implant depth as well as greater damage to the material in which it is implanted. This damage results in less channeling, and therefore shallower implanted profiles. However, there are significant problems associated with using BF.sub.2.sup.+ as a p-type dopant.
In particular, when fluorine is present in a gate conductor along with boron, fluorine enhances boron penetration through the gate oxide and into the channel region during thermal anneals. Boron which has penetrated into the silicon substrate may cause a shift in the threshold voltage (V.sub.th) of the operating device. This is because boron diffusion into the channel region results in a change in the concentration level of the n-channel substrate thereby causing a shift in threshold voltage and degrading oxide quality.
Accordingly, an efficient and practical solution to the problems associating with using BF.sub.2.sup.+ as a p-type dopant is desired.